The present invention relates to variable displacement hydrostatic transmissions and, more specifically to hydrostatic transmissions utilizing pumps and motors of the radial piston type.
A hydrostatic transmission transmits energy from rotary mechanical motion to fluid (typically oil) motion, and then back to rotary mechanical motion. Such transmissions are utilized in lawn and garden tractors, riding lawn mowers, and other small vehicles. Within a typical radial piston type hydrostatic transmission, incoming rotary mechanical motion rotates a cylinder disposed about a pintle. Pistons, positioned within the cylinders, are attached to slippers mounted in an expander band, so that as the cylinder unit rotates, the pistons are driven by the slippers which engage the surrounding eccentric annular track ring. U.S. Pat. No. 4,982,566 issued Jan. 8, 1991 to von Kaler et al. entitled "Expander Band for Hydrostatic Transmission," describes an expander band for a hydrostatic transmission in order to maintain the slippers against the track ring.
The pistons of the driving cylinder, or pump, create a pressurized fluid flow within the pintle which drives a receiving set of pistons likewise retained in a cylinder unit about the pintle. The receiving cylinder unit, or motor, drives an output shaft which, through gearing, drives the axles of the vehicle. The transmission ratio is thus directly proportional to the eccentricity of the track ring relative to the pintle. In the case of the pump, the eccentricity of the track ring relative the pintle is changeable such that a continuously variable ratio is achievable through reverse, neutral, and forward positions.
Prior art expander bands thus rotate with the cylinder block and pistons within the track ring. The rotation of the expander band creates additional turbulence in the fluid medium thus reducing the efficiency of the transmission. The rotating expander band also creates resistance to the rotation of the unit, thereby further reducing the efficiency of the transmission.
Conventional designs of expander bands include a plurality of slots circumferentially disposed at approximately equal distances. The slots allow the slippers to migrate from one end of the slot to the other as the cylinder unit rotates, preventing the pump or motor from binding up. However, the slippers tend to impact against and abrade the ends of the slots, which causes wear on the ends of the slots and slippers. This wear can affect the performance of the unit requiring repair or replacement. In addition, this wear can cause the ends of the slippers to chip away, causing metal particles to be introduced into the fluid. Such particles limit the efficiency and reduce the life of the transmission.
Further, prior art expander bands are generally split. As such, one end of the expander band can slide over the other end during operational rotation. This situation can cause inadequate fluid displacement of the pump and motor. It also can lead to binding of the hydraulic unit causing seizure thereof. Inadequate fluid displacement reduces the efficiency of the transmission, while seizure can require extensive repair of the unit.
What is therefore needed in the art is a way to mount the slippers of the pistons such that wear and abrasion are minimized. Also, the slippers must be positioned within the track ring such that no binding occurs. Further, the slipper mounting must be simple and economical.